Duplex printers and sheet transport systems for duplex printers that print both sides of a sheet and reference printing on each side of a sheet to a same edge of the sheet are known.
PCT application PCT/IL98/00553, which is incorporated herein by reference, describes a sheet transport system for a duplex printer that prints both sides of a sheet on a same impression roller. The publication also describes a sheet transport system for a tandem duplex printer that prints each sides of a sheet on a different impression roller.
The transport system, hereinafter referred to as a “re-feed transport system”, for the printer that prints both sides of the sheet on a same impression roller receives a sheet from the impression roller after a first side of the sheet is printed. If the second side of the sheet is to be printed, it turns the sheet over and feeds it back to the impression roller for printing the second side. The position of the sheet on the impression roller when the sheet's second side is being printed is registered to the position of the sheet's leading edge. If the second side of the sheet is not to be printed, the transport system moves the sheet to an output tray.
The transport system comprises a sheet transporter and a conveyor belt. The sheet transporter, hereinafter referred to as a “perfector”, operates to turn the sheet over and register the position of sheet on the impression roller to the leading edge of the sheet. The perfector comprises a first and second set of vacuum pick up arms. Each of the arms of the first set of arms is mounted to a same shaft and rotate with the shaft. Each of the arms of the second set of arms is mounted to an annulus that rotates about the shaft to which the arms of the first set of arm are attached. Each of the annuli is coupled by its own transmission belt to a drive shaft that rotates all the arms of the second set of arm together. The first and second sets of vacuum arms operate sequentially one after the other to repeatedly remove a printed sheet from the impression roller. When removing a sheet from the impression roller, a set of vacuum arms rotates in a direction opposite to the direction of rotation of the impression roller to a hand off position between the perfector and the impression roller. At the hand off position the set of vacuum arms grips the sheet by an edge of the sheet and rolls the sheet off the impression roller. If the sheet is printed on a first side, the edge that is gripped is a leading edge of the sheet, to which edge printing on the first side is referenced. If the sheet being removed from impression roller is to be printed on its second side, the set of vacuum arms places the sheet, printed side face up, on the conveyor and reverses its direction of rotation. Motion of the conveyor belt and the reverse rotation of the set of vacuum arms feed the sheet back to the impression roller trailing edge first. The set of vacuum arms grips the leading edge of the sheet until a gripper on the impression roller grips the trailing edge and the sheet begins to rolls up on the impression roller. As a result, the position of the sheet on the impression roller is registered to the leading edge of the sheet and printing of the second side of the sheet is referenced to the same leading edge to which printing on the first side of the sheet is registered.
PCT application PCT/IL99/00600 filed on 7 Nov. 1999 and entitled “Tandem Printing System with Fine Paper-Position Correction”, describes a sheet transport system and a sheet fine position control system for a tandem printer that prints each side of a sheet on a different impression roller. The disclosure of the application is incorporated herein by reference. The described sheet “tandem” transport system described herein is similar to the tandem sheet transport system described in PCT application PCT/IL98/00553.
The transport system of this application transports a sheet from a first impression roller of the printer, on which a first side of the sheet is printed referenced to a leading edge of the sheet, to a second impression roller of the printer, on which a second side of the sheet is printed. The transport system registers a trailing edge of the sheet to the leading edge so that the position of the sheet on the second impression roller is registered with respect to the leading edge. Printing on the second side of the sheet is therefore referenced to the same leading edge to which printing on the first side of the sheet is referenced.
The sheet transport system comprises a plurality of rotating sheet transporters. Each transporter comprised in the transport system comprises a rotatable shaft and preferably at least one array of suction cups that are mounted to the shaft for gripping a sheet. The transporter also preferably comprises at least one surface, hereinafter referred to as a “support surface”, for supporting a sheet that is gripped and held by the transporter's suction cups.
The transporters seriatim receive and hand off one to the other a sheet being transported by the transport system from the first impression to the second impression roller. A first transporter, hereinafter referred to as a “picker”, functions to remove a sheet from the first impression roller. The picker grips a leading edge of the sheet that it removes from the impression roller and hands off the leading edge to an adjacent transporter, hereinafter referred to as a “perfector”. The perfector turns over the sheet that it receives and hands off a trailing edge of the sheet to a next transporter, hereinafter referred to as a “transfer transporter”. The transfer transporter in turn hands off the trailing edge of the sheet to a feed roller that feeds the sheet to the second impression roller, which grips the sheet by the trailing edge.
In turning the sheet over, the perfector reverses its direction of rotation between clockwise and counterclockwise rotation and adjusts its speed of rotation so that when the trailing edge of the sheet is handed off to the transfer transporter, the trailing edge is registered to the leading edge. As a result, when the sheet is passed to the second impression roller, the sheet's position on the second impression roller is registered to the leading edge of the sheet, even though the second impression roller grips the sheet by the trailing edge of the sheet.